Q. What is a "Reverse Return" System?
A. A reverse return system is a type of closed loop system in which the return header is connected to the most hydraulically remote load, as shown in Figure 1. Compared to the direct return system in Figure 2 in which the return header is connected to the load closest to the pump, the reverse return system distributes the flows and pressures more evenly across the system, making it more inherently balanced.
Figure 1: Reverse return closed loop system.
Figure 2: Direct return closed loop system.The inherent balance of the reverse return system can be shown when modeled in PIPE-FLO and the systems are calculated. Let's first look at the pressure and flow distributions in the direct return system. Figure 3 shows the direct return system with no controls on the loads and the pump sized for 450 gpm (designed for 150 gpm through each identical load).
Figure 3: Direct return system calculated. Pump sized for 450 gpm.
The inlet pressure to each load decreases the farther the load is from the pump discharge, and the outlet pressure of each load decreases the closer the load is to the pump suction. This creates a larger differential pressure at Load 1 and a decreasing differential pressure across each load the farther the branch is from the supply pump. This differential pressure profile causes the flow rate to decrease from 155.9 gpm at Load 1 to 145.9 gpm at Load 3, a 10 gpm (or 6.4%) variation from minimum to maximum flow rate. The pressures and flow rates are summarized in Table 1 below.
Figure 4 shows the calculations for an identical system with the exception of an additional length of piping on the return header to create a reverse return system.
Figure 4: Calculated reverse return system. Pump sized for 450 gpm.
Just as with the direct return system, the inlet pressures to each load decreases the farther the load is from the pump. However, with the return header connected to Load 3, the outlet pressures decrease from Load 1 to Load 3 (opposite of the direct return system). This causes a smaller variation in the differential pressures across each load in the system. The inherent balance of this reverse return system produces a flow rate variation of 4.4 gpm, or just 2.9%.Table 2 summarizes the pressure and flow distribution in the reverse return system.
There are a couple of additional points to note about the calculated results for the two systems. Because the reverse return system requires an additional length of pipe at least the length of the return header, there is additional head loss that must be overcome by the head of the pump. This requires the total head of the pump in the reverse return system to be higher than the direct return system (147.9 ft vs. 129.7 ft in this example). Along with the added capital cost of the extra piping, the increased pump head results in higher operating cost, and may require a larger pump and motor to meet the demands of the system. Also, the increased pump head results in higher discharge pressures, which may affect the selection of pipe material or schedule and the capital costs of the piping.
The benefits of having an inherently balanced system may out-weigh the additional costs that may be incurred. Depending on the need for exact control of flow for each load, it may be possible to design the system without costly flow control valves and eliminate the associated controllers, wiring, pneumatic tubing, and other support instrumentation. An in-depth cost analysis should be done to determine the best solution for any given application.